A New Method for Constant Temperature Thermal Response Tests

Abstract

In ground source heat pump (GSHP) applications, determination of thermal properties such as thermal conductivity and diffusivity of ground is essential to make a reliable long term performance predictions of boreholes and system design. Constant heat-flux or constant-temperature thermal response tests (TRT) is used to determine ground s thermal properties. Constant-temperature TRT has some advantages over the constant heat-flux TRT due to its more accurate prediction, shorter test duration and compatibility with heat pump performance tests. In this study, an analytical method is developed for constant-temperature TRT to estimate thermal properties of a ground system of interest. Analytical solution of heat equation for a borehole is obtained based on Laplace transformation and heat transfer rate per unit borehole length (unit HTR) is analytically determined. Analytical expression for the variation of inverse unit HTR value with logarithm of time is fitted to experimental data to estimate thermal conductivity and diffusivity of ground. The advantage of this method is to allow to determine thermal conductivity directly from the slope of logarithmic time dependency of inverse unit HTR value without making an estimation for heat capacity. Therefore, errors based on incorrect estimation of heat capacity can be eliminated. The method is applied for different experimental data based on different test temperatures, and the results are compared with those of other methods. A very good agreement between the results is obtained. The method can be used to make more precise long term thermal performance predictions of boreholes.